1. Field of the Invention
This invention relates to an evaporator, and more particularly to an evaporator for a fuel cell system, whose size is small, weight is low and activating time is short. This invention also relates to a manufacturing process of the evaporator.
2. Description of the Related Art
It Air contamination caused by exhaust gases from automobiles is a critical problem. Electric vehicles have been suggested as a countermeasure for exhaust from automobiles. But electric vehicles have not become commercially viable due to the necessity of frequent charging.
In recent years, automobiles equipped with fuel cells generating electricity by reverse reaction of electrolysis using hydrogen and oxygen are being developed as an environmentally acceptable alternative vehicle, which does not discharge anything except water. A disadvantage of automobiles with this arrangement is that a heavy and bulky hydrogen gas cylinder or hydrogen reservoir made of alloy is required, which is not acceptable in automobiles.
Due to the reasons mentioned above, automobiles equipped with a fuel cell, and using hydrocarbon fuel, are considered to be the most promising environmentally acceptable vehicles. Methanol is the most applicable hydrocarbon fuel being used in fuel cells. A fuel cell of the above mentioned arrangement exhausts less wastes, other than carbon dioxide. The amount of discharged carbon dioxide from such a fuel cell is the same as that of an electric vehicle, taking account of the amount of carbon dioxide discharged when producing electricity at power plants. Introduction of fuel cells using hydrocarbon fuel is thus a countermeasure to global warming.
Aforementioned fuel cell generates electricity using a reformed gas whose principal element is hydrogen derived from steam and methanol (hydrocarbon fuel), and which is reformed using a catalyst (such as a Cu-Zn catalyst). Improving the efficiency of the vaporization of water and methanol improves the efficiency of the fuel cell. Particularly, for use in an automobile, an evaporator is required to be small in size and to evaporate the fuel as completely as possible.
Water and methanol vaporized by the evaporator are used as the fuel of the fuel cell. If the time required to activate the evaporator is shortened, the activating time for the fuel cell is also shortened.
As a prior art technique for evaporating a liquid, a device is shown in a Japan Patent Laid-open Publication H11-63429 (published on Mar. 5, 1999). This shows a water pipe group combustion type boiler in which many water pipes are disposed, and water in the water pipes is vaporized by heating the water pipes with combustion gas. However, this conventional evaporator has disadvantages. It reserves water (fluid to be heated) in water pipes in advance to ensure the resistance of the water pipe against combustion heat. In addition to the thermal capacity of the evaporator, since this structure requires a quantity of heat as latent heat of the fluid to be heated when activating, it takes a relatively long time to activate the evaporator.
A smaller air space in the water pipe, due to the space being occupied with the reserved fluid to be heated, lowers the boiling point in the evaporator of the conventional arrangement. This could cause the problem of incomplete combustion gas due to a halt of the combustion reaction on the surface of the water pipe. To achieve complete combustion, a large fuel space is needed. This further enlarges the size of the evaporator.
To prevent condensation of the vapor in the conduit conveying the vapor, and in equipment which utilizes the vapor such as a reformer converting vaporized fluid into reformed gas, superheated vapor which is heated to a predetermined high temperature higher than the boiling point of the vapor is required. In the conventional evaporator, vapor from the fluid to be heated reserved in the conduit remains saturated (which temperature is lower than superheated vapor). To obtain the superheated vapor, the conventional evaporator requires a superheating device. This enlarges the size of the evaporator.
Shell and tube type conventional evaporators need larger capacity in the heat transfer area for heat exchange. This also enlarges the size of the evaporator.
Accordingly, it is an object of this invention to reduce the size and weight of an evaporator.
It is a further object of this invention to shorten the activating time of the evaporator.
It is yet another object of this invention to provide an improved method for manufacturing the evaporator.
To solve the aforementioned and other problems, the evaporator of this invention includes a plate type heat exchanger comprising a plurality of piled plates defining a first conduit for a fluid to be heated and a second conduit for a heating fluid, the first and second conduits each having an approximately rectangular shaped section and being formed between said plates; and an atomizer mounted to supply the fluid to be heated to the first conduit.
The heat resistance of the heat exchanger is therefore ensured since the atomized fluid to be heated is supplied evenly in the entire interior of the conduit for the fluid to be heated.
The activating time can be shortened because there is no need to heat reserved fluid in liquid phase in the conduit for the fluid to be heated. Atomization of the fluid to be heated improves the thermal efficiency of vaporization because the increase of the surface area of the fluid to be heated facilitates vaporization.
A temperature gradient which distributes the high temperature at the supply side of the heating fluid and the low temperature at the supply side of the fluid to be heated is generated on plate portion due to the supply of the atomized fluid to be heated. This keeps the high temperature of the supply side of the heating fluid, reduces the influence of the reaction halt of the combustion gas, and makes it possible to achieve complete combustion in a smaller combustion space. These factors enable one to minimize the size of the evaporator.
Another aspect of this invention includes a first opening of the conduit for the fluid to be heated located on top surface of a plate type heat exchanger, a second opening of the conduit for the fluid to be heated located on a side portion of the plate type heat exchanger, an atomizer supplying the atomized fluid to be heated to said second opening, a third opening of a conduit for the heating fluid located on the bottom of the plate type heat exchanger, and a fourth opening of the conduit for the heating fluid located on the opposite side of the second opening, for supplying the heating fluid therethrough.
Accordingly, a positive temperature gradient in the vertically upward direction is generated because the fluid to be heated in the plate type heat exchanger is vaporized, high temperature vaporized fluid to be heated moves to the top part of the exchanger and the heating fluid moves to the bottom part of the exchanger. This generates superheated vapor since the vapor of the fluid to be heated is heated in a high temperature atmosphere.
A further aspect of the evaporator of this invention includes a fluid partition wall disposed in the middle of a conduit for the heating fluid for controlling the flow of the heating fluid in the vertical direction. Each plate of the heat exchanger can be heated evenly because the smooth flow of the heating fluid in the conduit separated by the fluid partition wall increases the heat transfer efficiency of the plate. This improves the thermal efficiency of the plate type heat exchanger and minimizes the size of the evaporator.
According to a further aspect of this invention, the plate type heat exchanger comprises a plurality of piled parallel heat exchanger modules, each of said modules comprising a U-shaped metal plate, and a flow blocking member positioned on an inside surface of said U-shape between said plates to block fluid flow at one end of said first conduit; a first interval maintaining member blocking the fluid flow towards top opening of the U-shaped member; and a second interval maintaining member blocking the fluid flow towards the sectional opening of the U-shaped member, the first and second interval maintaining members maintaining a predetermined interval between the piled parallel heat exchanger modules. There is no necessity of welding because of the structure of the plate formed by bending a metal plate, which makes the manufacturing process easier, reduces cost, minimizes the risk of leakage of fluid and gains heat resistance compared to the conventional plate type using two flat plates and a member maintaining the space with a predetermined interval between said two plates.
Another aspect of this invention includes an L-shaped member which includes the first interval maintaining member and the second interval maintaining member formed in one piece therewith, the first interval maintaining member blocking the fluid flow towards the opening of the top surface of the plate type heat exchanger and the second interval maintaining member blocking the fluid flow towards the opening of U shape on the side portion of the plate type heat exchanger. The structure of the interval maintaining members formed in one piece enables one to reduce the cost of manufacturing the interval maintaining members and to avoid leaks.
Still another aspect of this invention includes a metal plate being bent in a U shape, the first interval maintaining member blocking the fluid flow towards the opening of the top of the plate type heat exchanger as well as maintaining the predetermined interval between each plate, and the second interval maintaining member blocking the fluid flow towards the sectional opening of U shape on the side portion of the plate type heat exchanger and maintaining the space between each plate. Thus there is no necessity of welding because of the structure of this plate formed by a bent metal plate, which makes the manufacturing process easier, reduces the cost, minimizes the risk of leakage, and gains heat resistance compared to conventional plate type using two flat plates and a member supporting the space with a predetermined interval between the two plates.